The present invention has particular application to HBT MMICs (hetero-junction bipolar transistor monolithic microwave integrated circuits). An HBT MMIC is an integrated circuit using bipolar transistors, suitable for generation of relatively high radio frequency power levels. The present invention finds application in the transmitter path of a mobile phone handset. It is particularly suited to applications using linear modulation, such as the CDMA and W-CDMA schemes, as used in the emerging UMTS market.
HBT MMIC integrated circuits likely to be required in the transmit path of a handset include Driver Amplifiers, Variable Gain Amplifiers (VGA), and Power Amplifiers (PA).
Optimal RF performance of an HBT MMIC can only be achieved with a suitably designed bias circuit. The design of bias circuits for HBT MMICs needs to contend with at least some of the following problems.
The available battery voltage may be 3.0V or less. The typical base-emitter voltage of an HBT is 1.2-1.4V. This means that it is difficult to provide sufficient voltage to operate circuits having multiple ‘stacked’ HBT devices, which are ideally required.
The interaction of the RF signal with the bias circuit needs to be carefully addressed to ensure that RF performance is maintained over the full range of possible operating conditions, whilst maintaining DC efficiency.
To facilitate accurate setting of quiescent current levels, in the active bipolar devices, a regulated signal, most usually a voltage, is supplied to the HBT MMIC from the equipment. In the case of a mobile phone handset, the unregulated output of the battery Vcc which may vary from 3-6.5v, is regulated down to typically 2.8V±0.1V by additional circuitry within the mobile phone. This regulated signal is usually switched between 0V and 2.8V in order to provide power down signalling on the same line.
This regulator may supply several circuits within the mobile phone, and usually operates near the upper limit of its current supply rating. 1-5 mA may be available to the bias circuit on the HBT MMIC being described, and limiting current draw from this regulated line to a minimum level is a technical challenge. A MMIC with a high current draw from this regulated line will be less attractive to the market.
A power-down feature is also desirable in a battery operated device. HBT MMICs are often constructed in NPN arrangement only. In this case, it has been found difficult to design bias circuits which are capable of power-down to a very low current drain, without requiring a significant level of control current in the ‘on-state’.
Different bias arrangements are required for various RF circuits. A small-signal ‘class A’ circuit generally needs a bias circuit offering a good level of RF immunity, to avoid performance degradation due to self-biasing. A ‘deep class AB’ power amplifier circuit needs a high, but controlled, level of interaction between the active devices and the bias circuit under increasing RF drive level, if the required self-biasing profile is to be achieved.